The highly sensitive in vitro bioassay for measurement of luteinizing hormone (LH) and chorionic gonadotropin (hCG) in blood, developed in our laboratory and termed the RICT assay (rat Leydig cell testosterone bioassay), has been applied to the measurement of serum LH and LH-like gonadotropins in man and several animal species. This technique has indicated that the biological activity of LH secreted in man, rhesus monkey and rat is modulated by gonadal steroids. Studies on the pulsatile release of LH in men and postmenopausal women have demonstrated that LH is secreted in pulses of high biological activity. When the endogenous GnRH signal is amplified by opiate receptor blockade, bioactive LH is released in more frequent pulses of high biological activity, with a significant increase in androgen production. Long-term studies on the in vitro bioactivity of LH and CG in man and other species were continued. We have demonstrated in normal cycling women that biologically active LH is secreted in discrete episodic pulsations preferentially enriched in biologically active hormone, and that modulation of the frequency of bioactive pulses occurs during the late follicular phase of the menstrual cycle. This is an important physiological mechanism for regulating the blood concentration of LH available to the ovary and provides a basis for optimal treatment during the use of GnRH for the induction of ovulation. Our finding of significant discordance between immuno and bioactive LH pulsations indicates that estimates of bioactive LH are necessary to fully characterize physiological patterns of LH secretion during the menstrual cycle. We have also shown that the infertile mink is a useful model of human infertility, involving both endocrinological and immunological mechanisms. Our future goal is to define the effects of gonadal steroids and altered secretion rate upon the biochemical properties of pituitary and circulating LH, and to characterize the bioactive profiles of prolactin in cycling women.